1. Field of the Invention
This invention relates to optical fiber, and, in particular, to an optical fiber configuration for enhanced light amplification by stimulated Raman scattering. Accordingly, it is a general object of this invention to provide new and improved optical fibers of such character.
2. General Background
The Raman effect, as defined by the American Heritage Dictionary of the English Language, 1969, is: Physics. The alteration in frequency and random alteration in phase of light scattered in a material medium. [Discovered by Sir Chandrasekhara Venkata Raman (born 1888), Indian physicist.]
Light amplification by stimulated Raman scattering in fibers has been reported in publications including Y. Aoki et al., "Efficient Backward and Forward Pumping cw Amplification for InGaAs Laser Light in Silica Fibers", Electronics Lett., 19, 1983, pp. 620-622. Amplification factors exceeding 20 dB have been obtained in single mode fibers of several kilometers length with cw pump powers of typically one watt. The need for such large pump powers is a significant disadvantage.
The developed dB gain in the amplifier is inversely proportional to the area of the pump beam, generally proportional to the pump power and to effective length, and proportional to the Raman cross-section (a measure of the strength of the Raman interaction for the material) within the area of the pump beam.
The effective length L.sub.eff is equal to (1/.alpha.)(1-e.sup.-.alpha. L) where L=actual length in km and .alpha.=1/4.3 (fiber loss in dB/km).
The prior art has utilized silica fibers with GeO.sub.2 -doped cores. The Raman interaction in the fiber core increases with increasing germanium content, approximately proportional to the germanium concentration (the interaction in GeO.sub.2 is nine times stronger than that in SiO.sub.2). However, the GeO.sub.2 concentration was limited by the requirement that the fiber propagate only a single mode, since increasing such concentration increases core-cladding index difference and thus the number of modes. The core diameter can be decreased, decreasing the mode field diameter and increasing the Raman gain as in the fibers of the above-cited Aoki et al. reference but this can only be done to a limited extent because of losses due to increased fiber strain [B. J. Ainslie et al. "Monomode fiber with ultra-low loss and Minimum Dispersion at 1.55 .mu.m", Journal Electronics Letters, Vol. 18, no. 19, 1982, pp. 842-844] and excess losses in splicing to conventional fiber, thus limiting the increase in germanium doping. The previous art used fibers with SiO.sub.2 claddings. Disadvantageously, the propagating mode has a significant amount of light (tens of percent, typically) in the cladding, where it does not undergo Raman enhancement due to a GeO.sub.2 dopant.